The present invention generally relates to wideband microwave radio communications and, more particularly, to a circuit for removing atmospherically induced amplitude tilt distortion, i.e., distortion of amplitude depending on frequency, from microwave radio signals without requiring demodulation.
Distortions can significantly degrade the performance of digital communication systems such as those used in multi-gigabit per second (Gbps) satellite links with multi-giga Hertz (GHz) bandwidths, for example, or terrestrial air links using Quadrature Phase Shift Keying (QPSK) modulation or 16 Quadrature Amplitude Modulation (16 QAM) modulation. For earth to satellite communication systems the most common distortions may be classified into two categories: atmospheric distortions and hardware distortions. For a narrow-band signal, i.e., a signal with bandwidth less than approximately 500 mega Hertz (MHz), the atmospheric distortion is generally manifested as a time varying amplitude attenuation that is independent of frequency within the bandwidth of the signal. An exception in which the amplitude attenuation for a narrow-band signal may depend on the frequency is the multi-path effect, which can occur when the line-of-sight propagation path of the signal is close to the ground.
In contrast, for a wideband signal, i.e., a signal with bandwidth greater than approximately 500 MHz, the amplitude attenuation due to atmospheric distortion is typically frequency and time dependent. Atmospheric distortions, the most common of which are attenuations of amplitude depending on frequency, referred to as “amplitude tilt” or “amplitude tilt vs. frequency” distortions, are generally simple in nature, and can change rapidly in time. For wideband atmospheric communication links, severe rainstorms have been shown to cause linear amplitude tilt of many decibels (dB) that change at rates as high as 10 Hz. Hardware distortions generally are more complex in nature, may include phase changes as well as amplitude attenuations, and typically change at much slower rates.
The conventional approach to removing both atmospheric and hardware types of distortions from a communications link signal is to use a multi-tap transversal filter, typically with 10 to 20 taps. The approach of using a multi-tap transversal filter requires first demodulating a signal, i.e. detecting symbols in the signal, sampling the demodulated signal, using complex algorithms to make calculations based on comparing the detected symbols to known symbols in order to determine the tap weights, and providing feedback to the multi-tap transversal filter for eliminating the distortions. Typically, this process takes several seconds or longer to complete. Moreover, if the atmospheric distortions are large enough to prevent data detection, the communications link cannot be established.
Equalizing both atmospheric and hardware types of distortions with the same hardware requires a system complex enough to eliminate hardware distortions yet fast enough to eliminate atmospheric distortions, when, in general, a more complex system is slower and a faster system is simpler. Thus a simpler faster device for eliminating the more rapidly changing atmospheric distortions, i.e. amplitude tilt, which can be used in conjunction with the more complex hardware of the standard equalizer approach for removing the more complex, but less rapidly changing, hardware-induced distortions, is desirable. In addition, a device that can eliminate amplitude tilt from a signal, regardless of the bandwidth of the signal or the type of modulation, such as QPSK or QAM, is also desirable.
As can be seen, there is a need for a fast and simple way to remove the most common atmospherically induced distortion, i.e., amplitude tilt versus frequency distortion, from wideband microwave signals without requiring demodulation, or symbol detection. There is also a need for removing amplitude tilt that is simple and fast and can be used in conjunction with conventional approaches for removing hardware distortions. Moreover, there is a need for a fast and simple way to remove amplitude tilt distortion which is bandwidth and modulation independent, i.e., that is suitable for use in any wideband microwave link, using any modulation format, and where atmospheric distortions degrade performance.